Photonic crystals are periodical optical nanostructures that affect motion of photons in much the same way that ionic lattices affect electrons in solids. It was provided by E. Yablonovutch and S. John in 1987. For electromagnetic waves, energy band structures exist in a 3D-medium with periodically arranged dielectric constants. It is so-called photonic band gap system. In such structures, since the electromagnetic waves scatter in the periodical medium, some wave bands of the electromagnetic waves decrease exponentially due to destructive interference and can not transmit. Therefore, energy gap forms over the spectrum. Characteristics of propagation of the electromagnetic waves in the photonic crystals, including amplitude, phase, polarization direction and wavelength, can be significantly modulized by controlling characteristics of the photonic crystals, such as emitting spectrum, group velocity dispersion, polarization features, phase matching, etc.
Photonic crystals occur in nature in the form of structural coloration and are useful in different forms in a range of applications, for example, color changing paints and inks, photonic crystal fibers, optical fibers and optical computers. Because, photonic crystals are new optical materials for controlling and manipulating the flow of light, many recent inventions applied photonic crystal structures in light emitting diodes to enhance light extraction efficiency.
Please refer to FIG. 1. U.S. Pat. No. 8,288,755 disclosed a light emitting element with photonic crystal structure inside. The light emitting element has a substrate 100 to form all elements of the light emitting element on and pass light beams. A buffer layer 108 is formed on (Manufacturing processes are up side down. FIG. 1 only shows when the light emitting element works.) the substrate 100. Then, form photonic crystal patterns 106 and a pad pattern 107 in the same process. A light-emitting structure 110 is formed upon the photonic crystal patterns 106 and includes a first conductive layer 112, a light emitting layer 114 and a second conductive layer 116. An insulating layer 120 is formed on the upper surface and sidewalls of the light-emitting structure 110. A first ohmic layer 131 and a second ohmic layer 132 are created to fill openings formed by the insulating layer 120. Finally, a first electrode 140 and a second electrode 150 are produced to connect to external power. Therefore, when a bias (−) and bias (+) are applied to the first electrode 140 and the second electrode 150, respectively, light beams L are generated to emit upward, through the photonic crystal patterns 106. At this moment, the photonic crystal patterns 106 help more light beams out of the first conductive layer 112, i.e., increase light extraction efficiency.
The aforementioned invention utilizes the feature of photonic crystal which allows a specified band of light beams to pass through to enhance light extraction efficiency. Photonic crystal patterns are formed during the processes of a wafer. This is an application of photonic crystal structures with a wafer (or wafer level device). Since photonic crystals also have a characteristic to reflect specified light beams, if well arranged, the light beams can be reflected to show a specified color, even a specified logo. For some integrated circuits which have an open area to operate, for example sensing portion of a fingerprint reader, the reflected light be can indicate a direction to slide fingers or provide a trade mark for commercial use over the area. The present invention focuses a structure which fulfills the above goal.